This invention relates to differential pressure thermoforming apparatus and more particularly to sheet support apparatus for preventing undue sagging of a plastic sheet proceeding through heating apparatus to a forming mold station.
In continuous sheet, differential pressure, thermoforming machines, one of the problems encountered is that the material sags at a heating station before it reaches the mold station.
Thin sheets of thermoplastic material have relatively little "hot strength." When a thin sheet of thermoplastic material is clamped along its edges and is used for forming relatively large areas of parts, the strength of the heated, thermoplastic material is insufficient to support the midportion of the sheet.
One previous system for inhibiting sagging comprises the use of longitudinally extending, endless sag bands for supporting the midportion of the web or sheet in the manner disclosed in U.S. Pat. No. 2,967,328. One other practice which has been employed to inhibit sagging incorporates integral support strips or ribs in the sheet of plastic as disclosed in U.S. Pat. No. 3,664,791 issued May 23, 1972 to Gaylord W. Brown.
Presently the side edges of a plastic sheet are gripped and conveyed by advancing chains as described in U.S. Pat. No. 3,216,491 issued Nov. 9, 1965 for Plastic Sheet Conveying Apparatus. The continuous sheet or web of plastic is heated to thermoforming temperatures prior to reaching the mold station. Thermoforming machines are now being built wider than the previous machines to reach higher productive levels. The increased width of the theroforming machine and the increased sheet width has exaserbated the tendency of the sheets to sag under their own weight after reaching thermoforming temperatures. Accordingly, it is an object of the present invention to provide apparatus for supporting a portion of a sheet of plastic after it has been heated to thermoforming temperature but before the heated sheet portion reaches the mold station.
It is another object of the present invention to provide an air receiving, sheet support box directly underneath the plastic sheet in the last indexing station before the heated portion of the sheet reaches the mold station.
Still another object of the present invention to provide an air receiving, sheet support box which receives recirculated, pressurized fluid to support a portion of a formable plastic sheet in a reasonably planar condition while the sheet is receiving the final heat treatment, in the last heating station before reaching the mold.
It is a further object of the present invention to provide, underneath the sheet, sheet support apparatus comprising box frame means which receives pressurized, heated air for supporting the sheet.
A still further object of the present invention is to provide sheet support apparatus including a box frame which receives pressurized fluid and mechanism for controlling the temperature of the pressurized fluid at or slightly below the forming temperature of the plastic sheet being run. By controlling the temperature of the pressurized fluid at a temperature slightly below the forming temperature of the sheet it is possible to form a cooler skin on the underside of the sheet. This provides advantages when a mechanical plug assist is utilized in forming a deep drawn part. Accordingly, it is another object of the present invention to provide sheet support apparatus which will form a cooler skin on one side thereof.
It is yet another object of the present invention to provide sheet support apparatus for a continuous sheet differential pressure thermoforming machine including laterally extending, longitudinally spaced pressurized fluid receiving sheet support members for supporting longitudinally spaced portions of the sheet, and mechanism for introducing fluid to the longitudinally spaced sheet support members only while the sheet is being indexed.
A further object of the present invention is to provide a differential pressure thermoforming machine including laterally spaced apart, movable sheet support members for supporting the lateral edges of a continuous sheet and moving the sheet in a forward path of travel and stationary, pressurized fluid introducing mechanism interjacent the laterally spaced apart sheet support members for supporting the midportion of the sheet supported by the laterally spaced sheet support members.
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art as the description thereof proceeds.